We begin with the worldwide reaction today to a dangerous move in syria. Chemicals, dead ly gas loaded onto weapons near an airfield there. One drop could kill within minutes. So, world leaders are mobilizing tonight, deciding what they're going to do. And abc's senior foreign affairs correspondent martha raddatz takes us inside that story. Reporter: Today, hillary clinton, overseas, trying to tind some dip mroep matic way to end this increasingly dangerous conflict. 20 months of fighting, 40,000 lives lost. And now the chilling possibility of an air attack with deadly nerve agents. There is no question that we remain very concerned that the regime might very well consider the use of chemical weapons. Reporter: A senior u.S. Official saying that over the weekend, the syrian military loaded components of the nerve gas sarin into bombs on or near syrian airfields. They have not loaded the bombs onto aircraft, but the threat remains. Once these chemicals are poured into weapons, artillery shells, bombs that can be dropped from airplanes, they can be good up to almost two months. Reporter: The syrian government claims it will not use chemical weapons. But president assad is feeling the pressure from opposition forces who have gained strength and are now moving on the capital, damascus. Jeremy bowen, with our bbc partners, is there tonight and reports assad's forces are waging a fierce defense. Throughout the day and after dark, when I'm speaking to you, there are quite steady explosions of shell fire, outgoing artillery fire, going into the suburbs around the center of the city, where I am. Reporter: Residents are caught in a worsening cross fire. People who came from areas that are being shelled, they are on the streets, many children, sometimes you see them crying, old people are sleeping on the ground. Reporter: But nothing is this horrific war has gotten the attention of u.S. Officials more than this chemical weapons threat. These reports may mean that the united states and our allies are facing the prospect of an imminent use of weapons of mass destruction in syria. This may be the last warning we get. Reporter: I can tell you ning for all sorts of con tin jen sips is already well under way. But military action would not be easy. It could take upwards of 75,000 troops to secure those chemical weapons, which no one is eager to provide. But president obama, diane, has warned that using those weapons is a red line.

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